The present invention relates to a damascene structure and a CMP process for forming the damascene structure and, more particularly, to a structure of a barrier film on the upper surface of a Cu wiring and a CMP process for forming the barrier film structure. The invention is directed to achieve reduction in capacity between wirings, assurance of oxidation resistance and preventability of diffusion of Cu, and suppression of increase in wiring resistance.
A semiconductor device in recent years employs a damascene wiring (including a plug) structure from the viewpoints of size reduction and so on. In a multi-level damascene wiring structure, in order to prevent oxidation and diffusion of a metal filled in a groove, to provide an etching stopper and to reduce contact resistance, and so on, it is necessary to provide a cap film made of a material different from the material of the wiring on the upper surface of the wiring. It is important that the cap film is thin. One of the reasons is to prevent deterioration of the wiring performance (or plug performance), specifically, to minimize an RC delay. Another reason is that it is difficult from a process viewpoint to form a wiring (or plug) having a high aspect ratio, that is, a thick film.
The damascene structure will be described hereinbelow by using a wiring as an example. In a conventional technique, a groove for wiring is formed in an insulating film and is filled with a wiring material, thereby forming a wiring. After that, the wiring is etched only by a thickness corresponding to the thickness of a cap film to be formed, thereby forming a recess on the upper surface of the wiring. After that, the material of the cap film is deposited on the entire surface of the semiconductor substrate. By removing the cap film on a field region by CMP (chemical mechanical polishing), the cap film is left only in the groove for wiring. In such a manner, the cap film is formed by being embedded in the groove for wiring.
According to the conventional method, however, dishing is caused by the CMP in the cap film on the wiring and controllability with respect to the thickness of the cap film is low. For example, when the width of a wiring is set to 5 μm, although a cap film having a thickness of 40 nm is tried to be formed, as shown in FIG. 1, the thickness of the cap film becomes 15 nm in practice which is less than the half of the desired value. In an area where the wiring width is wider than 5 μm, the cap film formed is much thinner. That is, the wiring width dependency on the thickness of the cap film is large, the process controllability is low, and a desired film thickness cannot be obtained in an area where the wiring width is large.
In a further conventional technology, therefore, in the case of making the thickness of the cap film constant, a large amount of the recess is formed in the wiring, so that it is possible that a cap film having a desired thickness can be formed. According to the conventional method, however, the surface of a semiconductor substrate after the cap film on the field region is removed by CMP is very rough, and also the cap film remains on side wall of the groove. It causes the following problems. For example, a deviation occurs between the damascene wiring and a via hole formed on the damascene wiring, so that the contact material with which the via hole is filled is not sufficiently filled and the contact resistance increases. In the case where the cap film is conductive, the electrode area as a capacitor increases only by an amount corresponding to the residual cap film on the side walls of the groove. Consequently, the capacity between wirings increases and the RC delay of the wiring increases.
As described above, the conventional damascene structure and the CMP process for forming the conventional damascene structure have the problem caused by the dishing of the cap film as described in the first conventional example and the problem caused by the residual cap film on the side walls of a groove as described in the second conventional example. The problems obstruct improvement in characteristics. It is an object of the invention to provide a semiconductor device having an excellent RC characteristic by forming a cap film while avoiding the problems.
Hitherto, a TiN film, a TaN film, for example, are proposed to be employed as a film of preventing diffusion of Cu deposited on the surface of a Cu wiring. In order to realize the structure, the upper portion of the Cu wiring formed in the groove of the insulating layer is removed so that the Cu wiring is recessed from a level of the upper surface of the insulating film. Subsequently, a barrier film is formed on the entire surface of a semiconductor substrate. After that, the barrier film on the field region of the insulating film is removed by chemical mechanical polishing (CMP). The barrier film on the Cu film is, however, easily influenced by the state of the surface of the Cu film. It follows that that the diffusion preventability deteriorates in a rough portion caused by particles in the surface of the Cu film and a step portion between the insulating film and the wiring. In some cases, a defect occurs in the barrier layer by dishing and a damage at an edge portion at the time of CMP for forming the barrier layer. In order to ensure sufficient diffusion preventability even when a defect occurs, the barrier film has to be formed thick. Furthermore, the polishing speed of CMP varies in the surface of the wafer. The barrier film therefore has to be thick enough to ensure a high diffusion preventability and, in addition, to compensate the variation. Since such an increase in the thickness of the barrier film reduces the thickness of the wiring in the groove for wiring, effective wiring resistance (wiring resistance for the volume of the-groove) increases. Although it can be considered to deepen the groove in order to decrease the wiring resistance, in this case, the aspect ratio becomes higher. It increases not only the load of processing the groove and filling Cu but also the capacity between wirings since the facing areas of the wirings are enlarged.
It is therefore another object of the invention to provide a barrier film having the structure which can avoid the problems and to suppress diffusion of Cu into an insulating interlayer without increasing the capacity between wirings.